Low drift resistor structure for amplifiers

ABSTRACT

A low drift integrated circuit resistor structure has a forced high end and a forced low end. A sense high connection is located proximate to the force high connection, and a sense low connection is located proximate to the force low connection. The structure also has at least one internal sense connection. This structure can be used in an instrumentation amplifier that includes an operational amplifier which regulates the current between the force high connection and the force low connection in response, in part, to the current sensed in the internal sensing connection of the resistor structure. The sense high connection and the sense low connection form the outputs of the instrumentation amplifier.

This is a divisional of copending application Ser. No. 07/764,334, filedon Sep. 23, 1991 now abandoned.

TECHNICAL FIELD

The present invention pertains to resistor structures, and moreparticularly, to low drift resistor structures.

BACKGROUND OF THE INVENTION

Highly linear and stable instrumentation amplifiers are required incertain applications such as the amplification of signals fromthermocouples, strain gauges, and thermistors.

Present day amplifiers have a preset gain which varies with temperatureon the order of four or five parts per million per degree Celsius (ppm/°C). To provide some perspective as to the significance of a drift of 4ppm/° C., in a 12-bit digital system, one half of the least significantbit is equivalent to 128 ppm, and in a 16-bit digital system, one halfof the least significant bit is equivalent to 8 ppm. An amplifier with a4 ppm/° C. which operates in a temperature range of 0° to 70° C. canhave a drift of 280 ppm over the temperature range.

Such instrumentation amplifiers generally consist of operationalamplifiers and resistors. Since the gains of operational amplifiersavailable today is in excess of a million, the predominant driftproducing mechanisms are the gain setting resistors.

Therefore, it can be appreciated that a resistor structure whichprovides significantly less drift in an instrumentation amplifier typeof circuit is highly desirable.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide a resistorstructure in an instrumentation amplifier type of circuit which hassignificantly less drift than prior art resistor structures.

Shown in an illustrated embodiment of the invention is a low driftintegrated circuit resistor structure which includes a homogeneousresistive element formed in a first layer of the integrated circuit, theresistive element having a first end and a second end. The resistiveelement has a first sensing connection and a second sensing connection,the first sensing connection located proximate to the first end, and thesecond sensing connection located proximate to the second end. Theresistor structure includes at least one additional sensing connectionto the resistive element which is located between the first and thesecond sensing connections. At least one of the first, second, and theat least one additional sensing connection has a contact to at least oneadditional layer of the integrated circuit.

In a further aspect the resistor structure includes circuitry coupled tothe resistive element for passing a current through the resistiveelement. The circuitry is coupled to and responsive to the voltage on atleast one of the sensing connections, the circuitry controlling thecurrent through the resistor structure.

In another aspect, the circuitry includes a first operational amplifierhaving an output coupled to the first end of the resistor structure,with the first sensing connection forming a first output terminal. Thesecond end of the resistor structure is coupled to the output of asecond operational amplifier and the second sensing connection forms asecond output terminal. The at least one additional sensing connectionis coupled to a first input of the first operational amplifier, and afourth sensing connection to the resistive element, which is positionedbetween said at least one additional sensing connection and the secondsensing connection, is coupled to a first input of the secondoperational amplifier. A second input of the first operational amplifierreceives a first input signal, and a second input of the secondoperational amplifier receives a second input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other features, characteristics, advantages, andthe invention in general, will be better understood from the following,more detailed description taken in conjunction with the accompanyingdrawing in which,

FIG. 1A is a plan view of a prior art resistor structure;

FIG. 1B is a schematic diagram of an operational amplifier using theresistor structure shown in FIG. 1A as the gain setting resistors;

FIG. 2 is a first embodiment of a resistor structure according to thepresent invention;

FIG. 3 is an instrumentation amplifier schematic diagram which uses theresistor structure of the present invention;

FIG. 4A is a plan view of the preferred embodiment of the resistorstructure of the present invention;

FIG. 4B is a cross sectional view of a portion of the resistor structureshown in FIG. 4A.

It will be appreciated that for purposes of clarity and where deemedappropriate, reference numerals have been repeated in the figures toindicate corresponding features, and that the drawings of the resistorstructures have not necessarily been drawn to scale in order to moreclearly show the important aspects of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Upon examination of prior art resistor structures and by a series ofexperiments, it was determined that the main temperature and aging driftmechanism in the ratio of the resistors in the resistor structure arosefrom the contacts of the resistive element to the metal layer. Thepresent invention obviates the contact problem by controlling thecurrent through the resistor structure using sensing connections whichare low current connections and therefore not affected to anysignificant degree by the contacts in the sensing connections.

A resistor structure according to the preferred embodiment of thepresent invention has a first end or force high end and a second end orforce low end. Placed next to or proximate to the force high end is afirst sensing connection or sense high connection, and positioned closeto or proximate to the force low end is a second sensing connection orsense low connection. There is also at least one other sensingconnection located inside the resistor structure.

The contact drift problem is eliminated by forcing current through theresistive element between the force high and force low connections by acircuit which controls the current through the resistor structure inresponse to the voltage(s) appearing at the internal sensingconnection(s). The outputs from the resistor structure are formed by thesense high connection and the sense low connection.

Turning now to the drawings, FIG. 1 is a prior art resistor structure 10which includes a plurality of rows of resistive elements 12, 14, 16, 18,and 20. An amplifier minus input terminal on a metal line 22 isconnected to the left hand end of the resistive element 16 by ohmiccontact 24 and also to one end of the resistive element 20 by anotherohmic contact 26. The resistive elements 20 and 18 are connectedtogether by metallization through two ohmic contacts 28 and 30respectively. Similarly, the resistive elements 18 and 14 are connectedtogether by metallization through ohmic contacts 32 and 34 respectivelyand resistive elements 14 and 12 are connected together throughmetallization through two ohmic contacts 36 and 38. The other end ofresistive element 12 is coupled to an amplifier output terminal 40through an ohmic contact 42. The other end of resistive element 16 isconnected to ground through an ohmic contact 44.

FIG. 1B is a schematic diagram of an operational amplifier circuit 50which includes an operational amplifier 52 which receives an inputsignal VIN at its plus input terminal and provides an output signal VOUTat its output terminal. Coupled between VOUT and the minus input of theoperational amplifier are a series of resistors which consist of theresistive elements 12, 14, 18, and 20 shown as R12, R14, R18, and R20respectively. The resistor also includes the resistance of the ohmiccontacts shown as R42, R38, R36, R34, R32, R30, R28, and R26corresponding to the ohmic contacts shown in FIG. 1A. The minus input ofthe operational amplifier 52 is coupled to ground with the resistiveelement 16 shown as R16 in FIG. 1B and also through the ohmic contactsR24 and R44 which are the respective resistances of the contacts 24 and44 in FIG. 1A.

Therefore, the gain of the circuit 50 is shown as: ##EQU1##

It has been found that the structure shown in FIG. 1A, in which theresistive element is formed in polysilicon, that the gain drift (ΔG/ΔT)of this resistive element can be as much as 10 ppm/° C. for atemperature range of 25° C. to 125° C. and an additional 100 parts permillion per month at 125° C. ambient. The main gain drift mechanismarises from the contacts between the resistive element and themetalization layer.

A first embodiment, but not the preferred embodiment, of the presentinvention is shown in FIG. 2. FIG. 2 is a plan view of the resistorstructure 60 according to the present invention which includes aplurality of rows of equal widths of resistive material 62, 64, 66, 68,70, 72, 74, and 76. The rows of resistive material are connected atalternative ends by bridging bar connections 63 (between rows 62 and64), 65 (between rows 64 and 66), 67 (between rows 66 and 68), 69(between rows 68 and 70), 71 (between rows 70 and 72), 73 (between rows72 and 74), and 75 (between rows 74 and 76). Therefore, the resistorstructure 60 forms a serpentine pattern of homogeneous resistivematerial.

The top row, 62, of the resistor structure has a force high connection78 at the end of the top row having the bridging bar 63. The force highconnection 78 has an ohmic contact 80 to a metallization line 81. Asense high connection 82 is connected to the second row at the end ofthe second row having the bridging connection 63. The sense highconnection 82 has an ohmic contact 84 to a metallization line 85.

The bottom row 76 has a force low connection 86 which is connected tothe bottom row at the end of the bottom row connected to the bridgingbar 75. The force low connection 86 has an ohmic contact 88 to ametallization line 89. The sense low connection 90 is connected to thenext to bottom row 74 at the end of the row 74 connected to the bridgingelement 75. The sense low connection 90 has an ohmic contact 92 to ametallization line 93.

A first sensing connection 94 is connected to the row 66 at the end ofthe row 66 connected to the bridging bar 67. The first sensingconnection 94 has an ohmic contact 96 to a metallization line 97. Asecond sensing connection 98 is connected to the row 72 at the end ofthe row 72 having the connecting bar 71. The second sensing connection98 has an ohmic contact 100 to a metallization line 101.

In operation circuitry forces current along a main current path shown inFIG. 2 between the force high connection 78 and the force low connection86. The current through the main current path is regulated in part bythe voltage appearing at either the first sensing connection 94 or thesecond sensing connection 98 or a combination of both of these sensingvoltages. (The current through the main current path is also determinedby the input signal.) The outputs from the resistor structure shown inFIG. 2 is a voltage from the sense high connection 82 on metalizationline 85 and the sense low connection 90 on the metalization line 93. Thesensing connections 82, 90, 94, and 98, and their respective contacts84, 92, 96, and 100 are positioned so that they are removed from, oroutside, the main current path.

FIG. 3 is a schematic diagram of the preferred embodiment of the circuitto be used with the present invention. FIG. 3 shows an instrumentationamplifier 110 having a first operational amplifier 112 and a secondoperational amplifier 114. To improve the noise characteristics of theinstrumentation amplifier 110, the operational amplifiers 112 and 114are chopper stabilized in the preferred embodiment. The amplifiers arechopped by a chopping signal Fc shown in FIG. 3. The operationalamplifier 112 receives a positive input signal VINP at its plus inputterminal and has an output connected to the force high metalization line81. The minus input of the operational amplifier 112 is connected to themetalization line 97 of the first sensing connection 94. Themetalization line 85 of the sense high connection 82 forms the positiveoutput VOUTP of the amplifier 110.

The operational amplifier 114 receives a minus input signal VINM at itsplus input terminal and has an output connected to the force lowmetalization line 89. The metalization line 101 of the second sensingconnection 98 is connected to the minus input of the operationalamplifier 114. The metalization line 93 of the sense low connection 90forms the minus output signal VOUTM.

In operation the instrumentation amplifier 110 shown in FIG. 3 isdesigned to drive a signal into a differential input circuit having highimpedance inputs. Since each of the sensing contacts 84, 92, 96, and 100are coupled to high impedance nodes, virtually no current flows throughthese contacts and the contact resistance is insignificant. Since theresistance of the force high contact 80 and the force low contact 88 isinside the feedback loop of the operational amplifiers 112 and 114, theeffect of their resistance is negated by the feedback operation of theamplifiers. Therefore the gain of the instrumentation amplifier 110 is:##EQU2## where R2A=R64+R65+R66 and R2B=R72+R73+R74

and R1=R67+R68+R69+R70+R71

Therefore the gain of the operational amplifier 110 is dependent only onthe resistance of the homogeneous resistive element shown in FIG. 2 andis virtually independent of the contact resistances of the contacts 80,84, 88, 92, 96, and 100.

A test structure of the resistor structure shown in FIG. 2 was built andtested. This resistor structure provides a gain drift of approximately0.3 ppm/° C. for temperature spans of 25° C. to 125° C., and aging wasfound to be less than 10 parts per million per month at 125° C.

Note that in FIG. 2 the top row and the bottom row are not in thecurrent path and therefore do not perform a direct electrical functionof the resistor structure 60. However, the top row and the bottom roware useful during the formation of the resistor structure so that theetching seen by the rows 64 and 74 will be the same on both sides ofrows 64 and 74 as will be the same for all of the internal rows.

FIG. 4A is a plan view of the preferred embodiment of the resistorstructure according to the present invention. As shown in FIG. 4A, thepreferred embodiment of the resistor structure 130 includes many morerows than the resistor structure shown in FIG. 2. These additional rowsare necessary in order to provide the higher proportion of resistance ofthe feedback resistors R2A and R2B in relation to the bridgingresistance R1 between the operational amplifiers 112 and 114.

The resistor structure 130 is of polysilicon material and has a topmetallization layer plate 132 which lies on top of the resistorstructure and is connected to ground to provide shielding for theresistor structure. The resistor structure lies on the field oxideregion of the integrated circuit. Below the field oxide region under theresistor structure 130 is a pwel forming a second plate or shield underthe resistor structure. The pwel is also connected to ground.

In the preferred embodiment of FIG. 4A the distance from the force highcontact 80 to the sense high connection 82, and the distance from theforce low contact 88 to the sense low connection 90 is less than 5%, andpreferably less than 1%, of the distance of the main current path fromthe force high contact 80 to the force low contact 88.

A cross section of a portion of the resistor structure 130 is shown inFIG. 4B which shows the metal shield 132, the pwel 134, and the fieldoxide 136. The resistor structure is insulated from the metallizationlayer by an oxide layer 138.

The resistor structure 130 can also be formed in a diffused region inthe substrate or in a thin film layer.

Although the invention has been described in part by making detailedreference to a certain specific embodiment, such detail is intended tobe, and will be understood to be, instructional rather than restrictive.It will appreciated by those skilled in the art that many variations maybe made in the structure and mode of operation without departing fromthe spirit and scope of the invention as disclosed in the teachingscontained herein. For example, although the force and sensingconnections are shown in FIG. 2 as having contacts with themetallization layer, it will be understood that these connections couldalso be made through buried contact to source or drain regions oftransistors formed in the substrate of the integrated circuit. Moreover,if the resistive element is formed in the monocrystalline siliconsubstrate itself, then the resistive element could have connectionswhich consist of source and/or drain regions of transistors.

What is claimed is:
 1. A low drift amplifier comprising:(a) a firstoperational amplifier having an output coupled to a first end of acontinue homogeneous resistive element formed in a polysilicon layer ofthe integrated circuit; (b) a second operational amplifier having anoutput coupled to a second end of said continue resistive element suchthat current from said first end to said second end forms a main currentpath; (c) a first output of said low drift amplifier coupled to a firstsensing connection to said continue resistive element, said firstsensing connection located proximate to said first end; (d) a secondoutput of said low drift amplifier coupled to a second sensingconnection to said continue resistive element, said second connectionlocated proximate to said second end; (e) the negative input of saidfirst operational amplifier coupled to a third sensing connection ofsaid continue resistive element, said third sensing connection locatedbetween said first and second sensing connections; (f) the negativeinput of second operational amplifier coupled to a fourth sensingconnection of said continue resistive element, said fourth sensingconnection located between said first and second sensing connections;(g) said first, second, third and fourth sensing connections beingoutside said main current path; and (g) wherein the positive input ofsaid first operational amplifier receives a first input signal and thepositive input of said second operational amplifier receives a secondinput signal.
 2. The low drift amplifier recited in claim 1 wherein saidfirst and second operational amplifiers are chopper stabilized.
 3. Amethod for amplifying with low temperature drift first and second inputsignals comprising the steps of:(a) receiving said first input signal ata first input of a first operational amplifier; (b) coupling the currentfrom an output of said first operational amplifier into a first end of acontinue resistive element; (c) receiving said second signal at a firstinput of a second operational amplifier; (d) coupling the current froman output of said second operational amplifier into a second end of saidcontinue resistive element to form a main current path in said continueresistive element from said first end to said second end; (e) providinga first amplified output at a first sensing connection of said continueresistive element, said first sensing connection located near said firstend of said continue resistive element and outside said main currentpath; (f) providing a second amplified output at a second sensingconnection of said continue resistive element, said second sensingconnection located near said second end of said continue resistiveelement and outside said main current path; (g) coupling a signal from athird sensing connection of said continue resistive element to a secondinput of said first operational amplifier, said third sensing connectionlocated between said first sensing connection and said second sensingconnection and outside said main current path; (h) providing a signalfrom a fourth sensing connection of said continue resistive element to asecond input of said second operational amplifier; said fourth sensingconnection located between said first sensing connection and said secondsensing connection of said continue resistive element and outside saidmain current path.